Fix extension tests.

Stacks 3 different fixes for the testing:

* Use generator expressions to specify targets instead of hardcoding paths.  This requires updated syntax for the `add_test` command, so it was changed throughout.
* Use a launcher to wrap the executable, setting the environment and returning the exit code of the test.
* Convert tests into CLI modules using the SlicerExecutionModel, which has already accounted for the problem of loading the ITKFactory.  In one case I replaced the use of `metaCommand` with `PARSE_ARGS` and included the updated test parameters and associated xml file.  In most cases I omitted `PARSE_ARGS` and included a dummy xml file to satisfy SEM. 

Many thanks to @jcfr for all the help.